Total synthesis of halipeptins: isolation of halipeptin D and synthesis of oxazoline halipeptin analogues

The isolation from the marine sponge Leiosella cf. arenifibrosa and structural elucidation of halipeptin D (5), a relative of the previously isolated halipeptins A-C (1-3), is described along with the total synthesis of a number of oxazoline analogues (7 a-d and epi-7 c-d). The developed synthetic strategy provides a flexible entry into the various isomers of the polyketide domain of the halipeptins and improvises for a late stage construction of the oxazoline ring after a macrolactamization process which secures the required macrocycle.     

Synthesis and Duplex Stabilization of Oligonucleotides Consisting of Isonucleosides

Novel oligonucleotide analogues built from isonucleosides were synthesized by the phosphoramidite approach on an automated DNA synthesizer. The phosphoramidite building blocks were synthesized by phosphitylation of the corresponding protected isonucleosides. The oligonucleotide analogues C – G containing the isonucleoside 1 – 3 were studied with respect to their hybridization properties and enzymatic stability. The oligomers bearing an isonucleoside at the end of the strands all proved stable towards snake-venom phosphodiesterase, but only the oligomers D – G exhibit acceptable duplex stability when hybridized with complementary d(A₁₄).     

Total Synthesis of the Cyclic Depsipeptide Vioprolide D via its (Z)‐Diastereoisomer

The first total synthesis of vioprolide D was accomplished in an overall yield of 2.0 % starting from methyl (2S)‐3‐benzyloxy‐2‐hydroxypropanoate (16 steps in the longest linear sequence). The cyclic depsipeptide was assembled from two building blocks of similar size and complexity in a modular, highly convergent approach. Peptide bond formation at the C‐terminal dehydrobutyrine amino acid of the northern fragment was possible via its (Z)‐diastereoisomer. After macrolactamization and formation of the thiazoline ring, the (Z)‐double bond of the dehydrobutyrine unit was isomerized to the (E)‐double bond of the natural product. The cytotoxicity of vioprolide D is significantly higher than that of its (Z)‐diastereoisomer.     

Efficient synthesis of the A-ring phosphine oxide building block useful for 1 alpha,25-dihydroxy vitamin D(3) and analogues

The 1 alpha-hydroxy A-ring phosphine oxide 1, a useful building block for vitamin D analogues, was synthesized from (S)-carvone in nine synthetic operations and a single chromatographic purification in 25% overall yield. The synthesis features two novel efficient synthetic transformations: the Criegee rearrangement of alpha-methoxy hydroperoxyacetate 10 in methanol to obtain directly the desired secondary 3 beta-alcohol 11 and the highly chemo- and stereoselective isomerization of dieneoxide ester (E)-7 to the 1 alpha-allylic alcohol with an exocyclic double bond (E)-8. Further insight into the selectivity control of the latter rearrangement was obtained from the reactions of (Z)-epimeric substrates. The new synthetic approach leading to the 1 alpha-hydroxy epimers complements our previously reported synthesis of the corresponding 1 beta-epimers, thus producing all stereoisomers of these versatile building blocks efficiently from carvone.     

Enantiopure 4- and 5-aminopiperidin-2-ones: regiocontrolled synthesis and conformational characterization as bioactive beta-turn mimetics

Starting from aspartic acid, we synthesized lactam-bridged beta- and gamma-amino acid equivalents. Using the 1,4-bis-electrophile 1b as a central intermediate, the 4- and 5-aminopiperidin-2-ones 4 and 8, respectively, were approached by regioselective functionalization and subsequent lactamization. Diastereoselective C-alkylation was performed after N-protection of the lactam functionality when exclusive trans configuration resulting in the formation of 5a-f was observed in the 4-amino series. On the other hand, cis selectivity was typical for the alkylations of the 5-amino lactams 5a,b. To investigate the ability of the lactam building blocks to induce reverse-turn structures by intramolecular hydrogen bonding, the model peptidomimetics 12 and 14 representing Homo-Freidinger lactams of type II and III were prepared from 4a and 8a, respectively. Conformational analyses in dilute solution (1 mM) by IR and NMR spectroscopy at room temperature clearly indicated that the 4-aminopiperidin-2-one derivative 12 predominantly adopts a reverse-turn structure stabilized by a CO-HN hydrogen bond in an 11-membered ring. VT NMR experiments showed a substantial temperature dependency of the terminal NH when Deltadelta(NH)/DeltaT = -6.5 indicated that the amount of intramolecular hydrogen bonding is higher at low temperature. An application in the field of medicinal chemistry was demonstrated. Thus, starting from the Homo-Freidinger lactam 11c and the enantiomer ent-11c, we synthesized the peptidomimetics 15c and 16c and investigated them as lactam-bridged analogues of the dopamine receptor modulating peptide Pro-Leu-Gly-NH(2) (PLG). Both test compounds turned out to enhance significantly the agonist binding of dopamine D2 receptors, when the isomer 15c revealed a potency comparable to the genuine ligand PLG.     

Stereocontrolled and convergent total synthesis of amphidinolide T3

Stereocontrolled and convergent total synthesis of amphidinolide T3 has been described. A retrosynthetic scheme was constructed that led to the recognition of readily available and enantiomerically related compounds as starting materials for the total synthesis of amphidinolide T3. Thus, the two key building blocks 6 and 7 were defined as subtargets and synthesized in optically active forms. The C1-C12 fragment 6 was derived from commercially available D-glutamic acid or its synthetically equivalent (R)-5-hydroxymethyltetrahydrofuran-2-one 16 as starting material involving highly diastereoselective asymmetric allylation as a key step. The C13-C21 fragment 7 was efficiently synthesized in high yield through the dithiane coupling of the segment 10 and iodide 11, followed by subsequent deprotection and Petasis olefination. Eventually, assembly of the fragment aldehyde 6 and dithiane 7 along with C-C bond formation, a two-step oxidation-reduction sequence, selective macrolactonization, and functional transformation furnished the convergent total and formal synthesis of amphidinolide T3 and T4, and this approach also provides a flexible and practical synthesis of amphidinolide T macrolides.     

Cholesterol surrogates incorporating a benzophenone as part of the sterol tetracycle

Photoactivatable analogues 4-6 of cholesterol (1), having their cross-linking site in the ring D sterol region, have been synthesized starting from bromotetralone 14 via enantioselective Robinson annulation to enone 13 and Suzuki carbonylative coupling to the appropriate phenylboronic acid. Each of 4-6 was shown to substitute successfully for 1 in an assay of apo A-I-induced cellular cholesterol efflux, indicating that these analogues equilibrated with 1 in all major cellular pools.     

Synthetic studies of vitamin D analogues. IX. Synthesis and differentiation-inducing activity of 1 alpha,25-dihydroxy-23-oxa-, thia-, and azavitamin D3.

Three vitamin D3 analogues, 1 alpha,25-dihydroxy-23-oxavitamin D3 (3), 1 alpha,25-dihydroxy-23-thiavitamin D3 (4) and 1 alpha,25-dihydroxy-23-azavitamin D3 (5) were synthesized. In the differentiation-inducing activity of human myeloid leukemia cells into macrophages in vitro, the 23-aza analogue (5) showed the least activity, while no remarkable differences were observed between the 23-oxa analogue (3) and the 23-thia analogue (4), which were less active than 1 alpha,25-dihydroxyvitamin D3 (1).     

New approaches towards the synthesis of the side-chain of mycolactones A and B

New approaches towards the synthesis of the C1'-C16' side-chain of mycolactones A and B from Mycobacterium ulcerans are reported. Chiral building block 4 (Fig. 2) with the correct stereochemistry was obtained starting from naturally occurring monosaccharides, i.e. D-glucose or L-rhamnose. The polyunsaturated moiety 3 was synthesized in only 3 steps from 2,4-dimethylfuran. The building blocks were connected through a Sonogashira coupling resulting in the fast and convergent assembly of an 8,9-dehydro analogue 2 of the side-chain of mycolactones A and B. The synthesis of 1 is at this stage hampered by the lack of a selective partial hydrogenation protocol for alkynes embedded in a conjugated system. Alternative strategies involving palladium catalyzed sp2-sp2 coupling between C7' and C8' or C9' and C10' (Fig. 1) were also explored.     

Halipeptins A and B: two novel potent anti-inflammatory cyclic depsipeptides from the Vanuatu marine sponge Haliclona species.

Two new metabolites, named halipeptins A and B, have been isolated from the marine sponge Haliclona sp. Their structures were determined by extensive use of one- and two-dimensional NMR experiments, mass spectrometry, and UV and IR spectroscopy. Halipeptin A is a novel 17-membered cyclic depsipeptide, consisting of five residues including two alanines (with L stereochemistry) and three new residues that appear to be previously undescribed from natural sources: 1,2-oxazetidine-4-methyl-4-carboxylic acid, 3-hydroxy-2,2,4-trimethyl-7-methoxydecanoic acid (HTMMD), and N-methyl-delta-hydroxyisoleucine. The HTMMD residue is substituted with 3-hydroxy-2,2,4-trimethyl-7-hydroxydecanoic acid in halipeptin B. Halipeptin A was found to possess very potent anti-inflammatory activity in vivo, causing about 60% inhibition of edema in mice at the dose of 300 microg/kg (i.p.).     

Oligodeoxyribonucleotide analogues with bridging dimethylene sulfide,sulfoxide, and sulfone groups. Toward a second-generation model of nucleic acid structure

Short DNA analogues with bridging dimethylene sulfide, sulfoxide, and sulfone groups replacing the phosphate diesters (S-DNAs) were synthesized from building blocks prepared via two routes, both starting from D-glucose. Building blocks for RNA analogues were prepared by stereoselective introduction of nucleobase into a 2'-acylated ribose analogue. The ribose analogues were converted to deoxyribose analogues by replacement of a 3'-OH group by a thioacetyl unit, followed by photolytic deoxygenation or radical-based 2'-deoxygenation. DNA analogues joined via CH(2)(-)S-CH(2) units were prepared by S(N)2 displacement of a 6'-mesyl group on one building block using a thiolate nucleophile of another. 4,4'-Dimethoxytrityl protection and deprotection schemes were established for both the thiol and hydroxyl groups. The corresponding sulfoxide DNA analogues were obtained by oxidation with hydrogen peroxide. Sulfone DNA analogues were obtained by oxidation of the sulfide DNA with persulfate or hydrogen peroxide in the presence of a titanium silicate catalyst. The physical properties of several representative oligonucleotide analogues were examined, and interpreted in light of a "second-generation" model for DNA strand-strand recognition, a model that emphasizes the role of the polyanionic backbone in diminishing unwanted tendencies of highly functionalized molecules to form "structure" in solution. Even short sulfide-linked DNA analogues displayed association properties different from those displayed by standard DNA molecules. Complex formation observed with sulfide-linked tetramers by HPLC study in different solvents suggested that the complex is formed using hydrogen bonding. Sulfone-linked dinucleotides display Watson-Crick behavior; the tetramer, however, displayed self-structure. Self-structure and self-aggregation become more prominent as the length of the oligonucleotide analogues increases. The tendency to self-aggregate can be decreased by adding a charged sulfonate group to the 3'-end of the DNA analogue. Features of the second-generation model are important for many areas of nucleic acid chemistry, from the design of nucleic acid therapeutic agents to the search for life on other planets.     

Synthesis of chiral building blocks for cephalostatin analogues

The present paper describes the synthesis of 5‐azido‐6‐ketones (14) and 6‐hydroxy‐5‐ketone (20) from Hajos Wiechert ketone as chiral building blocks for cephalostatin analogues. The synthesis of symmetric cephalostatin analogue from 6‐hydroxy‐5‐ketone has also been reported. The characterization of the each synthesized compounds was carried out by IR, ¹H‐NMR, ¹³C‐NMR and High resolution Mass Spectrometry.     

Conformationally locked isostere of phosphoSer-cis-Pro inhibits Pin1 23-fold better than phosphoSer-trans-Pro isostere

Stereoisomeric cis and trans substrate analogues for Pin1 were designed and synthesized. The central phosphoSer-Pro core of the Pin1 substrate was replaced by cis and trans amide isosteres in Ac-Phe-Phe-pSer-Psi[(Z and E)CH=C]-Pro-Arg-NH(2), 1 and 2, peptidomimetics. They were synthesized on solid phase in 17% yield for the cis analogue 1, and 16% yield for the trans analogue 2. A second trans amide isostere with a C-terminal N-methylamide 3 was synthesized in 7% yield. The protease-coupled Pin1 assay showed that all three compounds inhibited the Pin1 peptidyl-prolyl isomerase (PPIase) enzymatic activity. The cis isostere 1 was 23 times more potent (K(i) = 1.74 +/- 0.08 muM) than its trans counterpart 2 (K(i) = 40 +/- 2 muM) in competitive inhibition of Pin1. These results suggest that the catalytic site of Pin1 binds cis substrates more tightly in aqueous solution. Antiproliferative activity toward the A2780 human ovarian cancer cell line by the cis and trans analogues correlates with Pin1 inhibition results.     

A practical synthesis of kifunensine analogues as inhibitors of endoplasmic reticulum alpha-mannosidase I

[reaction: see text] A practical synthesis of the potent class I alpha-mannosidase inhibitor kifunensine (1) beginning from the inexpensive and readily available starting material L-ascorbic acid (15) is described. The protected amino-alcohol ((2R,3R,4R,5R)-5-amino-2,3:4,6-diisopropylidenedioxyhexanol, 11) served as a key intermediate from which several N-1 substituted kifunensine analogues (including N-methyl, N-cyclohexyl, and N-bis(hydroxymethyl)methyl) and 2-desoxakifunensine analogues (including N-H and N-methyl) were prepared and screened for inhibition of human endoplasmic reticulum alpha-mannosidase I (ER Man I) and mouse Golgi alpha-mannosidase IA (Golgi Man IA). In addition, several pseudodisaccharide kifunensine analogues in which a mannose residue was tethered to N-1 of kifunensine via a two-, three-, or four-carbon linker and an affinity-bound kifunensine analogue were also prepared and evaluated for biological activity. While the synthesized N-1 kifunesine analogues were found to be less potent inhibitors of Class I alpha-mannosidases than kifuensine itself, the bis(hydroxymethyl)methylkifunensine analogue 6 was shown to selectively inhibit ER Man I over Golgi Man IA.     

Stereoselective Synthesis of D‐ and L‐Carbocyclic Nucleosides by Enzymatically Catalyzed Kinetic Resolution

An efficient synthesis of (S)‐ or (R)‐3‐(benzyloxy‐methyl)‐cyclopent‐3‐enol was developed by appling an enzyme‐catalyzed kinetic‐resolution approach. This procedure allowed the syntheses of the enantiomeric building blocks (S)‐ and (R)‐cyclopentenol with high optical purity (>98 % ee). In contrast to previous approaches, the key advantage of this procedure is that the resolution is done on the level of enantiomers that only contain one stereogenic center. Owing to this feature, it was possible to chemically convert the enantiomers into each other. By using this route, the starting materials for the syntheses of carbocyclic D ‐ and L ‐nucleoside analogues were readily accessible. 3′,4′‐Unsaturated D ‐ or L ‐carbocyclic nucleosides were obtained from the condensation of various nucleobases with (S)‐ or (R)‐cyclopentenol. Functionalization of the double bond in 3′‐deoxy‐3′,4′‐didehydro‐carba‐D ‐thymidine led to a variety of new nucleoside analogues. By using the cycloSal approach, their corresponding phosphorylated metabolites were readily accessable. Moreover, a new synthetic route to carbocyclic 2′‐deoxy‐nucleosides was developed, thereby leading to D ‐ and L ‐carba‐dT. D ‐Carba‐dT was tested for antiviral activity against multidrug‐resistance HIV‐1 strain E2‐2 and compared to the known antiviral agent d4T, as well as L ‐carba‐dT. Whilst L ‐carba‐dT was found to be inactive, its D ‐analogue showed remarkably high activity against the resistant virus and significantly better than that of d4T. However, against the wild‐type virus strain NL4/3, d4T was found to be more‐active than D ‐carba‐dT.     

Synthesis of a 1α-C-methyl analogue of 25-hydroxyvitamin D3: interaction with a mutant vitamin D receptor Arg274Leu

Vitamin D₃ analogues have been developed for a mutant vitamin D receptor (VDR), Arg274Leu. The mutant VDR has a mutation at Arg274, which forms an important hydrogen bond with 1α-OH of 1α,25-dihydroxyvitamin D₃ to anchor the ligand tightly in the VDR ligand binding pocket. Stereoselective synthesis of the A-ring part of the novel vitamin D analogue, 2α-(3-hydroxypropyl)-1α-methyl-25-hydroxyvitamin D₃ (4), from d-galactose was accomplished with the key steps of the introduction of the methyl and allyl groups to the chiral building blocks. The new analogue 4 is ca. 7.3-fold more active than the natural hormone 1α,25-dihydroxyvitamin D₃ (1).     

Synthesis and evaluation of macrocyclic transition state analogue inhibitors for alpha-chymotrypsin

Lactams 1 and 2 are readily formed from acyclic precursors in the presence of trypsin and chymotrypsin, respectively, identifying the macrocyclic ring system as a potential motif for constrained transition state analogue inhibitors of the serine peptidases. Ketone 3 was synthesized and shown to be a modest inhibitor of chymotrypsin (Ki = 220 microM), albeit 4-fold more potent than the acyclic hydroxy acid 25 (Ki = 1.5 mM as a mixture of epimers). A precursor (31) to the amino boronic acid 4 was also prepared; although this derivative was a potent inhibitor of chymotrypsin (Ki = 130 nM) by virtue of the boronic acid moiety, it showed no advantage over the des-amino analogue 32 (Ki = 120 nM), which is not capable of cyclizing.     

C1-C2 cleavage of C1-functionalized 8-oxabicyclo-[3.2.1]-oct-6-en-3-one. Stereoselective preparation of 4-substituted butenolides

Diastereoisomeric epimers at C1′ of 4-(1-methyl-2-oxo-butyl)-2-butenolide, and the corresponding saturated γ-lactones, were synthesized by hydrolysis of 1-methoxy-8-oxabicyclo[3.2.1] oct-6-en-3-one, under acidic conditions. These butenolides are interesting synthetic building blocks, precursors of biologically active natural products like insect pheromones. Their formation could be explained by a cleavage at the C1-C2 bond of the oxabicyclic precursor. On the basis of the experimental data we have proposed a mechanism of hydrolytic cleavage which is formally an intramolecular reverse Dieckmann process.     

Bisamides derived from azulene-1,3- and -5,7-dicarboxylic acids as new building blocks for anion receptors

Bisamides based on the azulene moiety were investigated as building blocks for anion receptors. In the course of these studies, derivatives of azulene-1,3- and -5,7-dicarboxylic acid were synthesized and thoroughly characterized. The anion affinities of the derivatives based on functionalization in the five-membered ring and in the seven-membered ring were determined by (1)H NMR titration. The structural analysis of these building blocks was performed by X-ray diffractometry, molecular modelling and 2D NMR spectroscopy. The five-membered ring derivatives are easy to obtain, offer a binding site preorganized in the syn-syn conformation and bind anions with a strength similar to those of pyrrole-based analogues. There is also strong evidence for aromatic CH...anion interactions. The ligands substituted at the 5- and 7-positions offer a binding cleft with an uncommon geometry that originates from the seven-membered ring and seems to be complementary to the chloride anion.     

Duplex and triplex formation of mixed pyrimidine oligonucleotides with stacking of phenyl-triazole moieties in the major groove

5-(1-Phenyl-1,2,3-triazol-4-yl)-2'-deoxycytidine was synthesized from a modified CuAAC protocol and incorporated into mixed pyrimidine oligonucleotide sequences together with the corresponding 5-(1-phenyl-1,2,3-triazol-4-yl)-2'-deoxyuridine. With consecutive incorporations of the two modified nucleosides, improved duplex formation with a complementary RNA and improved triplex formation with a complementary DNA duplex were observed. The improvement is due to π-π stacking of the phenyl-triazole moieties in the major groove. The strongest stacking and most pronounced positive influence on thermal stability was found in between the uridine analogues or with the cytidine analogue placed in the 3' direction to the uridine analogue. Modeling indicated a different orientation of the phenyl-triazole moieties in the major groove to account for the difference between the two nucleotides. The modified oligonucleotides were all found to be significantly stabilized toward nucleolytic degration.